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Whoever becomes Mexico’s next president will have to overcome intense social instability as a result of recent “negative” campaigning, a top aide to Andrés Manuel López Obrador, the leftwing candidate, has warned.

Manuel Camacho, a congressman for Mr López Obrador’s Democratic Revolution party (PRD) and one of his campaign’s architects, claimed that the aggressive advertising strategy adopted recently by Felipe Calderón of the centre-right National Action party (PAN) had “polarised society”.

“The middle class has become divided and the popular classes are very angry now. There are going to be demonstrations on a scale rarely seen in Mexico, and that will make it very hard to govern, no matter who wins,” he told the FT in an interview.

With a little over a month before Mexicans vote, most political analysts are once again predicting a tight result. Polls over the last month have shown Mr Calderón catching and even passing Mr López Obrador for the first time since the campaign began.

A poll published on Wednesday in the Reforma daily newspaper still shows Mr Calderón ahead of Mr López Obrador – although his lead has narrowed to just four points compared with seven in the same poll a month ago.

Political commentators say much of Mr Calderón’s recent surge is the result of a controversial – and costly – advertising campaign aimed at linking Mr López Obrador with economic mismanagement and radical figures such as President Hugo Chávez of Venezuela.

On Tuesday Mexico’s Electoral Tribunal ruled against several PAN adverts in a decision that could severely limit Mr Calderón’s ability to mount similar campaigns in the future. His team said it would appeal against the decision.

Barely two weeks before the main candidates fight it out in a potentially decisive live televised debate, Mr Camacho acknowledged Mr Calderón’s improved position in the polls although he added that it was “just a bubble”.

But while confirming that his candidate and party would respect whatever the result of the July 2 election, Mr Camacho labeled Mr Calderón’s tactics “irresponsible” and accused him of running a campaign “based on lies”.

One of them, he said, involved claims that Mexico City’s debt had tripled during the period in which Mr López Obrador was the city’s mayor. In fact, it had only grown from $28bn to $40bn.

Mr Camacho also criticised Vicente Fox, Mexico’s president and a member of Mr Calderón’s party, for “his constant intromissions in the campaign”. Although strict electoral laws forbid public officials from backing a particular candidate, Mr Fox had “been on the campaign trail [for Mr Calderón] for months”.

The Federal Electoral Institute (IFE) this week enforced a ban on public officials announcing or inaugurating infrastructure projects or social programmes until after the election, and said they would publicly reprimand Mr Fox if he did not respect the ban.

Mr Camacho said: “The extent to which the president has already got involved in the election would be enough to annul it [under the country’s laws]. Every day he supports Calderón and attacks Andrés Manuel.”

Meanwhile, late Tuesday, members of Mr Calderón’s campaign said they would appeal a ruling by Mexico’s Electoral Tribunal against three advertisements in which the PAN attacked Mr López Obrador.

Mr Calderón’s team withdrew the adverts weeks ago but the ruling, if allowed to stand, would likely severely hinder a second round of negative campaigning against Mr López Obrador in the final run-up to the election.

Mr Camacho confirmed that Mr López Obrador’s campaign had recently doubled its thus far modest advertising budget in preparation for the final phase of the electoral race.

But he ruled out launching a retaliatory campaign against Mr Calderón. Instead, he said, it would appeal to core values and “remind the Mexican people who Andrés Manuel really is”.